Machines which convey webs typically include a number of sequential process sections, for example, a photographic film or paper coating machine may have several coating sections, setting sections, and drying sections operative upon one or both sides of a web during a single pass of a web through the machine. Typically, each process section has its own web tension regime and benefits from being isolated in tension from the sections immediately preceding and succeeding it.
In order for a web to move smoothly through such a machine, and to unwind into and wind out of the machine, the web must be under tension at all times. However, many of the conveyance elements in a machine, especially rollers, exert inertial drag on a web, and after passage over many rollers a web will exhibit a loss in tension. Therefore, drive rollers which are responsive to tension sensors are included at intervals through the machine to change or restore tension to a selected level. Thus there may be a drive roller at the end of one section and/or another drive roller at the start of the next section.
Instabilities in web tension between these drive rollers require a dynamic means to variably lengthen or shorten the web path. Such means and its action between tension-restoring machine elements is referred to herein as "tensioning." Typically, this is provided by a device in the web path consisting of two rollers mounted parallel to each other on a common frame, with the first side of the web fading one roller and the second side of the web facing the other roller. The frame pivots about an axis parallel to and midway between the rollers and transverse to the direction of web advance in order to accumulate or pay out web as needed. A torsion device acting about the axis applies a couple to the frame, which tensions the web variably in response to a web tension sensing device. U.S. Pat. Nos. 2,685,417; 2,714,268; and 4,496,113 all show web tensioning devices comprising two parallel rollers rotatable about an axis that is between and parallel to the roller axes for absorbing and releasing web material to control tension.
In tile conveyance of a web, there also exists a need to position the web controllably in the cross-web direction, sometimes repetitively, through a machine. Without such control, a web will wander laterally in a machine due to the cumulative effect of minute misalignments of conveyance components and variations in the straightness and planarity of the web itself. Alternately, succeeding sections of the machine may not be co-linear, through design or error. The lateral positioning of a moving web in a web conveyance machine is herein referred to as "steering."
Steering is typically accomplished by pivoting a frame carrying rollers, similar to the tensioning apparatus just described, about an axis in the plane and direction of the incoming web and tangential to the first roller in the frame. Such as device is disclosed in U.S. Pat. No. 4,069,959. Pivoting this frame displaces the outgoing web laterally, but introduces, and in fact requires, a twist in both the incoming and outgoing webs. The device serves as a steering device for a web when coupled to a downstream web lateral position sensor and an appropriate actuator and feedback system.
Twist in the incoming and outgoing webs in the machine may be undesirable. One approach in dealing with twist has been to isolate twist from the main web conveyance by leading the web past a high-wrap fixed roller, around a pair of parallel air conveyance elements commonly known as "air bars" which are mounted in a frame pivotable about an axis orthogonal to the axis of the first air bar, and past a second high-wrap fixed roller. Such a device is available from The Kohler Coating Machinery Corp., Greentown, Ohio, U.S.A.
All of these prior tension isolation and steering devices suffer from loss of web traction at high web speeds due to entrainment of a thin layer of air between the rollers and the web. The loss of web traction results in a reduction in steering control and is especially troublesome for webs having low-friction coatings. Prior art devices require separate apparatus for tensioning and for steering, leading to large, space-consuming, expensive installations. Prior art devices also require very precise alignment between the rollers in the tensioning and steering frames, as well as between the frames and adjacent upstream or downstream rollers, to avoid creasing, scratching, or scuffing of webs and especially very thin webs. Such alignment is difficult and expensive to provide, for example, in applications in which rapid changeover or maintenance is required between product runs. Failure to provide such alignment in prior art conveyance machines can result in damaged or defective product, increased manufacturing cost, and decreased customer satisfaction.